What makes plants different? Principles of extracellular matrix function in 'soft' plant tissues

An overview of the biomechanic and morphogenetic function of the plant extr acellular matrix (ECM) in its primary state is given. ECMs can play a pivot al role in cellular osmo- and volume-regulation, if they enclose the cell h ermetically and constrain hydrostatic pressure evoked by osmotic gradients between the cell and its environment. From an engineering viewpoint, such c ell walls turn cells into hydraulic machines, which establishes a crucial f unctional differences between cell walls and other cellular surface structu res. Examples of such hydraulic machineries are discussed. The function of cell walls in the control of pressure, volume, and shape establishes constr uctional evolutionary constraints, which can explain aspects commonly consi dered typical of plants (sessility, autotrophy). In plants, 'cell division' by insertion of a now cell wall is a process of internal cytoplasmic diffe rentiation. As such it differs Fundamentally from cell separation during cy tokinesis in animals, by leaving the coherence of the dividing protoplast b asically intact. The resulting symplastic coherence appears more important for plant morphogenesis than histological structure: similar morphologies a re realized on the basis of distinct tissue architectures in different plan t taxa. The shape of a plant cell is determined by the shape its cell wall attains under multiaxial tensile stress. Consequently, the development of f orm in plants is achieved by a differential plastic deformation of the comp lex ECM in response to this multiaxial force (hydrostatic pressure). Curren t concepts of the regulation of these deformation processes are briefly eva luated. (C) 2000 Elsevier science Inc. All rights reserved.